The invention relates to a method for operating an internal combustion engine with variable gas exchange control times, particularly for rapidly reaching the operating temperature during cold starting, for the temporarily intensified generation of hot gas and/or for implementing a multistroke operating mode with an increased number of strokes per working cycle, the latter particularly in order to apportion the combustion process to a plurality of working strokes of a working cycle. The term xe2x80x9cgas exchange control timesxe2x80x9d refers to the control times of the inlet and outlet valves, designated as gas exchange valves, of the combustion space or combustion spaces of the internal combustion engine.
The development of freely activatable gas exchange valves, i.e., of inlet and outlet valves, which can be placed into their open or closed position at variably predeterminable points in time during the various working strokes, as described, for example, in German Published Patent Application No. 195 01 495, has made it possible, for the purpose of achieving specific desired effects, to set unconventional gas exchange control times differing from those which are normally used during customary two-stroke or four-stroke operation.
One problem which can be tackled by this technique of variable gas exchange control times is that of reaching the operating temperature as rapidly as possible during a cold start, in order to keep the exhaust-gas emissions as low as possible during the warming-up phase and/or to make it possible, for use in the vehicle engine, to make a sufficient quantity of heat available for heating the vehicle. This is based on the notion, when the internal combustion engine is cold, of not converting the energy contained in the fuel into drive energy in an uncompromisingly optimum manner, but, instead, in particular periods of time, such as after a cold start, also to use it deliberately for heat generation, in order to make corresponding accessories, such as stationary heating appliances, superfluous. This should, of course, be accompanied by exhaust-gas emissions which are as low as possible.
A further requirement during the operation of many internal combustion engines is a temporarily intensified generation of hot gas, for example for the turbine of a coupled exhaust-gas turbocharger. In this application, it is, for example, expedient to improve the transient behavior of turbocharged engines, i.e., engines with an exhaust-gas turbocharger, at the transition from the part-load to the full-load operating mode, this transient behavior being designated as a so-called xe2x80x9cturboholexe2x80x9d and being relatively poor, as compared with aspirating engines.
German Published Patent Application No. 1 034 925 describes a method for operating a piston internal combustion engine with an exhaust-gas turbine, in which it becomes possible to set variable gas exchange control times by an adjusting control which makes it possible, by an associated lever being actuated, to vary the angular position of a camshaft in relation to a crankshaft. When, starting from an instantaneous operating state, the drive power supplied by the internal combustion engine on an output shaft is to be reduced, an advanced adjustment of the outlet valves is performed by the adjusting control, in order to continue to feed essentially the same energy to the exhaust-gas turbine, so that it can be maintained in an operating range with high efficiency. In this internal combustion engine, the exhaust-gas turbine can be coupled to its output shaft serving, for example, for driving a motor vehicle or, together with a corresponding supercharger part, can form an exhaust-gas turbocharger. In both cases, there may be provision, particularly in the low rotational speed range, by the advanced adjustment of the outlet valves to feed a large amount of energy to the exhaust-gas turbine, so that the latter can generate a relatively high torque.
In order to achieve a part-load operating mode which is beneficial in terms of fuel consumption, the technique of so-called cylinder cut-off or fade-out is conventional, in which, in a predeterminable part of the successive working cycles, some of the plurality of combustion spaces of a multicylinder internal combustion engine are deliberately xe2x80x9ccut outxe2x80x9d or xe2x80x9cfaded outxe2x80x9d, in that, contrary to normal operation, no fuel injection occurs, so that the cut-off cylinders merely xe2x80x9cfollowxe2x80x9d passively. This arrangement is accompanied by suitable activation measures for the gas exchange valves. Operating methods of this type are described in German Published Patent Application No. 44 40 920, European Published Patent Application No. 0 703 357, German Published Patent Application No. 42 92 543 and U.S. Pat. No. 5,655,508.
It is an object of the present invention to provide an operating method, by which, utilizing the possibility for setting variable gas exchange control times, an internal combustion engine may be operated relatively favorably in terms of consumption and with low pollutant emission, precisely even in special operating situations, such as for warming up and for the intensified generation of hot gas.
The above and other beneficial objects of the present invention are achieved by providing an operating method as described herein. The operating modes may be provided in corresponding operating situations for one and the same internal combustion engine, the possibility of having the capability of setting variable gas exchange control times being utilized. It is also possible, in any given internal combustion engine, to implement only one of these various operating modes, e.g., in combination with a conventional two-stroke or four-stroke operating mode, or to permit any desired choice of these operating modes for the internal combustion engine.
The method according to the present invention includes a direct exhaust-gas recirculation operating mode, in which a retarded adjustment of the outlet valve or outlet valves is performed, i.e., as compared with normal operation in which the outlet valve is opened only during a gas expulsion phase, it is closed, with a delay, only during a gas intake phase following the gas expulsion phase. As in all other operating modes according to the present invention, this arrangement may occur in each working cycle or only for a selectable part of the successive working cycles of one or more selectable combustion spaces, i.e., the respective operating mode may be set individually for each combustion space and each working cycle according to a predeterminable combustion-space and/or working-cycle pattern. As a result of this arrangement, part of the previously expelled hot exhaust gas is sucked directly back into the combustion space again. The hot exhaust-gas fraction heats the fresh-air quantity subsequently fed via the inlet valve, so that, overall, the discharge of heat to the combustion space walls increases. This operating mode is therefore particularly suitable, for example, for reaching the operating temperature more rapidly during a cold start.
In the direct exhaust-gas recirculation operating mode, the respective inlet valve may be opened, delayed by about the same delay time as the outlet valve in relation to normal operation, i.e., the opening action for the inlet valve then alternates in the conventional manner with the closing action for the outlet valve. This arrangement avoids an appreciable exchange of gas between the intake tract and the exhaust tract.
In the direct exhaust-gas recirculation operating mode, advanced adjustment of the outlet valve may be additionally provided, i.e., the outlet valve is not only closed later than normal, but is also opened earlier than normal, i.e., even before the commencement of the gas expulsion phase during a preceding combustion and expansion phase. The effect of this arrangement is that hot exhaust gas which is still under pressure expands into the outlet tract, so that less energy is converted into mechanical work, and therefore additional exhaust-gas heat occurs, which may be utilized in order to reach the operating temperature more rapidly. Moreover, if required, a still relatively energy-rich or enthalpy-rich hot gas may be provided by the early opening of the outlet valve, in order to bring about an increased power output during a following expansion process, for example, at an exhaust-gas turbine of a turbocharger.
The method according to the present invention may include an exhaust-gas holding operating mode, in which, for at least part of the successive working cycles, the outlet valve is not opened in the usual manner after a combustion and expansion phase, but, instead, is kept closed during a subsequent compression phase and is opened only in the course of a subsequent expansion phase or at the end of the latter and, consequently, at the commencement of a subsequent gas expulsion phase. This insertion of an additional compression and expansion phase increases the number of strokes of the working cycle by two strokes, i.e., in a four-stroke engine, six-stroke operation is obtained, and has the result that, as compared with normal operation, the still hot exhaust gas dwells correspondingly longer in the combustion space and therefore more time is available for the transmission of heat to the combustion space walls. This operating mode is also consequently suitable for rapidly reaching the operating temperature for the internal combustion engine itself and, if appropriate, for a connected exhaust emission control system and, especially in motor vehicle applications, also for the increased provision of heat for heating a vehicle interior. In this exhaust-gas holding operating mode, the energy of the burnt fuel is not converted into mechanical drive energy in an uncompromisingly optimum manner, but is utilized for deliberately providing exhaust-gas heat favorably in terms of consumption and with low pollutant emission.
The operating method according to the present invention may include an exhaust-gas filling operating mode, in which, for at least part of the successive working cycles, the outlet valve is not closed at the end of a gas expulsion phase, but is kept open beyond the subsequent gas intake phase, and at the same time the inlet valve also remains closed in the gas intake phase. The combustion space is thereby filled solely with exhaust gas in this gas intake phase. Subsequently, a compression and expansion phase, with gas exchange valves kept closed and with ignition and fuel injection remaining deactivated, is inserted. The energy necessary in this period of time for this follow-up of the cylinder must be applied in another manner, for example, by other cylinders fired in the same period of time with a higher load than is conventional. In this case, increased wall heat transmission may occur in these cylinders operated with a higher load. The compression energy occurring in the compression phase during exhaust-gas compression may be partially discharged to the combustion space wall and partially converted into gas pressure. By the early opening of the outlet valve even during the expansion phase, some of the gas pressure may be converted into increased exhaust-gas heat, instead of into mechanical drive work. This exhaust-gas filling operating mode is therefore also suitable for the effective provision of heat, particularly also during the warm-up of the internal combustion engine.
The method according to the present invention may include an additional-stroke operating mode, in which the combustion process is apportioned to a plurality of strokes of the corresponding working cycle and/or an additional exhaust emission control working stroke is inserted, for which purpose the working cycle contains corresponding additional working strokes. Thus, between two gas intake phases with a fully open inlet valve, for example, two combustion and expansion phases with a preceding compression phase may be provided, in which the gas exchange valves in each case remain closed and the mixture located in the combustion space is ignited. Individual fuel injection into the combustion space for each of these combustion and expansion phases may be performed, as required. In particular, two directly successive combustion and expansion phases, in each case with an associated compression phase, may be provided within one working cycle. As a further possibility, an exhaust emission control measure, for example, the addition of reducing agent which reduces nitrogen oxides, may be provided in an additional working stroke.